Definitions

1
Definitions

• Primitive appearing earlier in the fossil
  record, ancestral character state
• Derived appearing later in the fossil record as
  a new evolutionary innovation, descendant
  character state
• Clade all organisms within a single
  evolutionary lineage stemming from a common
  ancestor (can also be called a taxon)

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Definitions (cont.)

• Homology specific organs or structures in
  different animal groups that have descended, with
  modification, from similar organs present in
  ancestors (e.g., mammal limb from fish fin)
• Homologous structures in different animals result
  from inheritance from a common ancestor and arise
  from a common portion of the genome, so they can
  be used as evidence of evolutionary relatedness
• Analogous structures organs or structures
  having similar functions but not similar ancestry
  (e.g., wings of insects and birds fish tail fin
  and whale fluke)
• Cant be used as evidence for evolutionary
  relatedness

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Classification General Features

• Basic Unit of Classification Species
• Biological Species Concept a group of
  similar-looking individuals capable of
  successfully interbreeding
• Phylogenetic Species Concept a genetically
  distinct group reproductively isolated from other
  such groups (i.e., gene flow restricted)
• All concepts treat species as independent and
  closed genetic systems

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Classification General Features

• Species are grouped into higher phylogenetic
  (taxonomic) units as well
• Each successively higher unit contains fewer and
  fewer shared characteristics
• Therefore, the higher the taxonomic unit, the
  less closely related are the organisms belonging
  to that unit
• Goal of classification is to provide a correct
  phylogeny evolutionary family tree of living
  and extinct organisms

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Derived Characters
Primitive Character
Hooves
Fig 1.27 Classification Example
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Classification System (General to Specific)

• Classification System
• Domain (General category, few shared traits)
• Kingdom
• Phylum
• Class
• Order
• Family
• Genus
• Species (many shared traits)

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Classification General Features

• Relationships among higher clades (or at least
  which organisms belong to which clades) generally
  pretty well understood, but relationships among
  lower phylogenetic clades less well understood
• Classification is dynamic, changing as new
  information is discovered
• Currently, much revision in phylogenetic
  relationships is associated with recent molecular
  work (gene sequencing)

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Chordate Origins

• 2 groups of organisms with a bilateral body plan
  (if split in two, get two halves that are mirror
  images)
• Deuterostomes blastopore (an opening occurring
  in early embryo) becomes anus includes
  Echinoderms, Hemichordates and Chordates.
• Protostomes blastopore becomes mouth includes
  Molluscs, Annelids, and Arthropods.
• Recent molecular data have confirmed this
  division, which was originally based on
  differences in embryology.

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Fig 2.3
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Chordate Origins

• Phylum Chordata includes vertebrates and
  invertebrate chordates
• Allied by the 4 chordate morphological
  characteristics (present in all chordates at some
  point during life cycle)
• Notochord stiff supporting rod along dorsal
  part of body, underneath dorsal nerve cord
• Pharyngeal gill slits
• Dorsal hollow nerve cord
• Post-anal tail

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General Chordate Characteristics
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Chordate Origins

• In addition to the morphological shared features
  (dorsal nerve cord, notochord, pharyngeal gill
  slits, postanal tail), all chordates have cells
  that produce thyroid hormones (endostyle or
  thyroid gland).
• Endostyle glandular groove in floor of pharynx
  involved in filter feeding
• Thyroid hormones (tyrosine precursor include
  bound iodine) necessary to maintain adequate
  growth, development, tissue differentiation and
  metabolic rate.
• Evolutionary Trend from clusters of cells (in
  endostyle) to discrete gland (thyroid)

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Chordate Origins

• Related Phylum Hemichordata includes
  Pterobranchs (Class Pterobranchia) and acorn
  worms (Class Enteropneusta)
• Pterobranchs tiny, rare marine animals that
  form plant-like colonies individuals project
  like small flowers at the end of a secreted tube
  (test)
• Possess one pharyngeal gill slit, but no other
  chordate characteristics
• Filter feed by ciliary action

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Chordate Origins

• Acorn worms elongated worm-like animals that
  burrow in tidal mud flats
• Anterior end with a proboscis (used in burrowing)
  followed by a collar (contains mouth). These
  structures also present in Pterobranchs.
• Possess well-developed pharyngeal gill slits, but
  no truly homologous structures to other chordate
  characteristics
• Also show a dorsal nerve cord (more or less
  hollow) in collar region, but this becomes more
  diffuse posteriorly (not a complete cord)
• At base of proboscis is stomocord stout patch
  of support tissue (not homologous to notochord)

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Chordate Origins

• HOX genes (genes involved in early development)
  produce reversed dorsal and ventral body pattern
  between chordates and all other animals,
  including Hemichordates

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Acorn Worm
Figs 2.8 2.14 Hemichordates Acorn worms and
Pterobranchs
Pterobranch
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Invertebrate Chordates

• Phylum Chordata consists of 3 Subphyla
• Urochordata tunicates or sea squirts
• Cephalochordata lancelets (amphioxus)
• Vertebrata vertebrates (or Craniata comprised
  of hagfish vertebrates)
• Hagfish lack vertebrae around their notochord, so
  they have been separated out from the rest of the
  vertebrates in some classification schemes
• Recent evidence suggests that hagfish have
  secondarily lost vertebral elements, so
  Vertebrata is current name for Subphylum

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Urochordates (Tunicates)

• Marine animals that may be solitary or colonial
• Adults have sac-like shape with outer covering a
  leathery tunic (cellulose)
• Filter feeders using pharyngeal gill slits
  adults lack other chordate characteristics
• Larvae are free-swimming and have all four
  chordate characteristics
• Larvae metamorphose into sessile adults
  notochord, dorsal nerve cord and post-anal tail
  degenerate

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Figs 2.22 and 2.23 Urochordate larvae
and metamorphosis
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Fig 2.24
Adult Urochordates
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Cephalochordates (Amphioxus)

• Elongated, vaguely fish-like marine animals that
  live mainly buried in sand or silt with the head
  region protruding
• They are capable of swimming
• Filter feeders using pharyngeal gill slits
• Possess all 4 chordate characteristics as an
  adult so the most clearly related to the
  vertebrates among the invertebrate chordates,
  although genetic evidence suggests that they are
  the most basal of the chordates.
• Possess segmentally arranged muscle masses
  (myomeres) in the lateral body walls. Similar to
  common condition in vertebrates
• No paired fins present, only metapleural folds on
  ventral region of the body (lack of paired fins
  is similar to primitive fishes)

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Fig 2.16
Cephalochordates
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Vertebrata (Vertebrates)

• Characterized by cephalization concentration of
  nerve cells (into a brain) and sense organs in
  the anterior (head) region.
• Necessary because vertebrates are active animals
  and need to perceive information from the front
  of the body as they move forward.
• Possess vertebral column (backbone)
• Specialized kidney tubules
• Includes Fishes, Amphibians, Reptiles, Birds,
  Mammals

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Chordate Origins

• General trend in Chordate evolution increasing
  levels of activity, and to some extent,
  increasing body size
• Chordate Ancestry
• Echinoderms (sea urchins, starfish) share
  similarities in embryonic development and larvae.
• Hemichordates and Chordates perhaps evolved from
  a common ancestor, possibly like present-day
  echinoderm (Garstangs Hypothesis) or
  hemichordate larvae via paedomorphosis (
  retention of larval characteristics in sexually
  mature adult).
• Recent molecular evidence offers little support
  for this view.

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Fig 2.30 Garstangs Theory of the Origin of
the Chordate Body Plan
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Chordate Origins

• Current View Genes involved in determining
  dorsal-ventral body plan function to produce
  opposite body plan in chordates and echinoderms
  hemichordates
• This inverted body plan was a major evolutionary
  innovation, but why it occurred is not known.
• In any event, the chordate ancestor was probably
  a mobile bottom-dweller as an adult, that fed by
  ciliary/mucus feeding system
• Development of pharyngeal gill slits improved
  feeding characteristics.
• Other chordate characteristics likely arose to
  enhance locomotion abilities in chordate
  ancestors.

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Fig 2.32
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Vertebrate Origins

• Vertebrate Ancestry
• One hypothesis (Garstangs) proposes that larval
  form of urochordate (free-swimming with all 4
  chordate characteristics) that normally
  transformed into a sessile adult, instead became
  sexually mature in the larval stage, or while
  retaining larval morphology ( paedomorphosis)
• This retention of larval characteristics in a
  sexually mature stage occurs in one group of
  present-day Urochordates (Class Larvacea), so
  there is precedent for such a change.
• The advantage of this lifestyle is that the
  organism could actively seek out favorable
  foraging opportunities.
• This organism then likely led directly to
  Vertebrates (Cephalochordates are a sister clade)

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Vertebrate Origins

• Alternative Hypothesis
• A prechordate may have abandoned sedentary
  filter-feeding lifestyle to become actively
  predaceous.
• This lifestyle favors development of
  chordate-like characteristics notochord,
  muscular tail, dorsal hollow nerve cord.
• Urochordates and Cephalochordates reversed this
  trend (reverted to sessile filter feeders), but
  vertebrates did not (continued toward active
  foraging lifestyle).

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Evolutionary ScenarioChordate/Vertebrate Origins

• Early pre-chordates likely mobile,
  bottom-dwelling worms similar to Acorn Worms.
• Pharyngeal gill slits evolve to aid in
  ciliary/mucus system of feeding.
• Increases in locomotion to promote more active
  foraging may have led to appearance of other
  chordate characteristics that favor more
  efficient movement.

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Evolutionary ScenarioChordate/Vertebrate Origins

• These changes led to primitive amphioxus-like
  body form
• Incipient predator with better differentiated
  head, pharyngeal gill slits, eyes, expanded mouth
• OR
• Active suspension feeder
• From this ancestor invertebrate chordates and
  vertebrates diverged
• Secondary loss of mobility to less active
  filter-feeding system (Urochordates and
  Cephalochordates)
• Increase in activity and better developed
  predatory traits (Vertebrates)
• Muscular pumps form to assist with filter feeding
  and allow for formation of gills for breathing in
  vertebrates

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Early Vertebrate Evolution

• Ciliary, suspension-feeding pre-vertebrate
  (likely somewhat similar to amphioxus)
• Evolutionary development of muscular pharyngeal
  pumps and cartilage support of pharyngeal arches
• Agnathan fish (lack jaws) using muscular pumps to
  create currents for filter-feeding
• Evolutionary development of jaws allowed feeding
  apparatus that could select individual food
  particles of larger size
• Gnathostome fish with jaws and active selection
  of food items (predation)